Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 143.7	$ 137.7	$ 141.0
Other comprehensive income:
Amortization of net actuarial losses, net of $15.1 million, $13.4 million, and $7.6 million of income taxes in 2021, 2020, and 2019, respectively	46.0	40.3	29.5
Actuarial gains (losses), net of $26.5 million, $10.4 million, and $6.9 million of income taxes in 2021, 2020, and 2019, respectively	80.5	(31.5)	(26.5)
Amortization of prior service costs, net of income taxes	0.1	0.1	(0.1)
Other comprehensive income, net of income taxes	126.6	8.9	2.9
Comprehensive income	$ 270.3	$ 146.6	$ 143.9